Semiconductor devices are increasingly being used as input devices for digital systems. For example, in identification and security applications, semiconductor devices are used to provide user identification information. One such device is a semiconductor fingerprint sensor.
FIG. 1 shows a portion of a typical semiconductor fingerprint sensor 100. Generally, such a sensor is provided as an integrated circuit (IC). The sensor 100 includes a die (or wafer) 102 attached to a substrate 104 via an adhesive or epoxy bond 106. A sensor surface 108 of the die 102 has a conductive grid, shown in detail at 110, that is used to form a capacitive circuit to detect characteristics of a person's finger when the sensor surface is touched. The grid is coupled to a plurality of die contact members 112 at the surface of the die.
A technique known as wire bonding is used to couple the die contact members 112 to substrate contacts 114 located on the substrate material, which is normally made up of metallic lead frame or build up layers of substrate. Typically, wire bonding involves attaching small wires (gold or aluminum) between two contact members. A capillary device, shown at 116, is typically used to bond the wire between the contacts. When bonding the wire, the capillary device first forms a ball 118 at the end of a wire 120 by using an electronic flame-off (EFO) technique. Once the ball is formed, the capillary device attaches the ball 118 to a die contact pad 112 by a thermal-sonic process. In this process, the contact is heated and ultrasonic power is used to agitate the ball onto contact to flatten out the ball to form an inter-metallic weld between the ball and the contact, as shown at 122.
After the first weld is made, the capillary device 116 extends the wire 120 over to a substrate contact 114 to form a weld with that contact. To bond the wire to the substrate contact 114, a stitch weld is formed. The stitch weld bonds the wire to the substrate contact and cuts the wire at the same time, so that the capillary device may form a new ball on a next portion of the wire and proceed to the next die contact. For example, a stitch weld is shown at 124.
The wire 126 shows the result of the wire bonding process described above. Because the wire extends in generally a vertical direction from the weld of the ball to the die contact, a wire loop is formed when the wire is extended to the substrate contact. The wire loop has a height above the surface of the die is shown at 128. For standard wire bonding processes, this loop height is between six to ten thousandths (mils) of an inch high. As described in the following text, the loop height has an effect on the operation of finger sensor 100.
Once the wire bonding is completed and all bonding wires are installed, the device is protected by an encapsulation process in which a material, such as plastic, completely covers the bonded wires. For example, a molding process may be used where a material is molded around the device. Another process that may be used is referred to as “glob-top” dispensing, where material is dispensed onto the top of the device and allowed to flow around the sides and bottom of the device.
FIG. 2 shows the finger sensor 100 after an encapsulation process is completed so that the bonding wires are completely protected by an encapsulation material 202. However, for the finger sensor to operate, the sensor surface 108 is exposed by a cavity 204 in the encapsulation material to allow a person finger to come in contact with the sensor surface.
To cover the bonding wires and still provide access to the sensor surface 108, the cavity in the encapsulation material includes cavity walls 206 that are at least as high as the loop height of the bonding wires. The cavity walls form what is referred to as a pedestal that has a pedestal height, shown at 208. Unfortunately, as a result of the pedestal height, portions of the sensor surface 108 may not be reachable by a person's finger. For example, the sensor surface regions shown at 210 and 212 may be inaccessible to a person's finger because it is not possible squeeze the finger into the corner formed by the sensor surface and the cavity wall.
Finger sensors typically provide their best operation when a maximum number of grid points can be touched. However, due to the effects of the pedestal height, portions of the sensor grid are unreachable, and so, the performance of the sensor may be degraded. Another problem associated with convention fingerprint sensors is the package size. Typical fingerprint sensors have die contacts on either side of the sensor surface. This results in a very large package that may be unsuitable for use in portable applications.
One way to overcome the above problems is to provide a larger cavity to account for the unreachable portions of the sensor surface. However, due to the geometry of the die, it may not be possible to provide a larger cavity without exposing portions of the die. Furthermore, even if a larger cavity were possible, the overall height of the encapsulation is undesirable because typical applications for finger sensors include portable devices, such as cell phones, that require the smallest possible size. For example, one conventional fingerprint sensor has approximate dimensions of 22×12×0.4 millimeters, which is a relative large package that is unsuitable for use in portable applications.
Therefore, what is needed is a way to provide maximum access to a finger sensor surface while providing the smallest possible size to allow the device to be used in a variety of portable applications.